Ethylene, alone or with other olefinic unsaturated monomers are frequently polymerised in the presence of catalyst compositions, which have essentially two components: a compound of a transition metal belonging to any of groups 4 to 6 of the Periodic Table of Elements (IUPAC 2007) which is often called a procatalyst, and a compound of a metal belonging to groups 1, 2 or 13 of said Table which is the s.c. cocatalyst. This kind of Ziegler-Natta catalyst composition has been further developed by depositing the procatalyst on a less or more inert and particulate support and by adding to the catalyst composition in the stages of its preparation several additives, among others electron donating compounds.
The catalyst composition used for the production of the ethylene (co-)polymers determines i.a. the properties of the polymers. Thus, the catalyst composition allows for a “tailoring” of the properties of the produced resins.
For tailoring the properties of the produced ethylene polymers it was already suggested to use ZN-catalysts comprising an internal donor, like THF (tetrahydrofuran).
For example WO 00/58374 discloses ZN catalysts comprising THF, whereby the primary purpose of the THF is to reduce the level of electrostatic charge in the polymerisation medium so that agglomeration of polymer particles does not occur.
U.S. Pat. No. 5,055,535 discloses a method for controlling the MWD of polyethylene homopolymers and copolymers using a ZN catalyst comprising an electron donor selected from monoethers (e.g. tetrahydrofuran). The monoether, like tetrahydrofuran is added to the catalytic component and the cocatalyst, at the latest, upon commencement of the polymerisation reaction and that under no circumstance should the monoethers be brought into contact with the catalytic component without the presence of the cocatalyst in the medium.
EP 1780225 A1 suggest the possibility of tailoring the properties of a multimodal ethylene polymer by using a modified ZN catalyst to influence the molecular weight distribution (MWD) of a higher molecular weight (HMW) component whilst essentially having no affect on the MWD of the lower molecular weight (LMW) component. The electron donor present in the ZN catalysts used is preferably tetrahydrofuran, whereby the ZN catalyst is preferably pre-formed and then contacted with an electron donor and optionally dried.
Nevertheless, although much development work has been done in the field of Ziegler-Natta catalysts for producing polyethylene, there are still some problems to be solved.
One problem that is often encountered with prior art Ziegler-Natta-catalysts is that it is difficult to produce an ethylene homo- or copolymer having high molecular weight. Polyethylenes of high molecular weight show i.a. an enhanced toughness. However, as the molecular weight of the polyethylene increases, the processability of the resin usually decreases.
Further nowadays HSE-(health, safety & environment) policies are an important factor in the production of catalysts and further polymers. In other words the polymers must fulfill the strict health and environmental requirements national and international institutions. One class of substances which have been considered as potential harmful compounds is phthalates, which have been commonly used as internal electron donors in Ziegler-Natta type catalysts. Also tetrahydrofuran has been recognized as a hazardous substance.
For these reasons it is still desirable to find alternative internal donors which do not include phthalates and/or tetrahydrofuran and which yields desired polymer properties, namely high molecular weight.
Thus, it was an objective of the present invention to provide a Ziegler-Natta procatalyst and its manufacture, which is environmentally sustainable and supports the preparation of ethylene polymers with high molecular weight.
Surprisingly these objects could be solved by the use of a supported Ziegler-Natta procatalyst with a special bi-(oxygen containing ring) compound as internal donor, which is added during the catalyst preparation.
Such bi-(oxygen containing ring) compounds have the formula (I)

wherein in the formula (I)
X is C or Si,
R1 can be the same or different and can be hydrogen, a linear or branched C1 to C8-alkyl group, or a C3-C8-alkylene group,
and R2 to R5 are the same or different and can be hydrogen, a linear or branched C1 to C8-alkyl, or a C3-C8-alkylene group,
whereby two or more of R1 to R5 can form a ring,
n is the same for both oxygen-containing rings and can be 0, 1 or 2,
whereby the two oxygen-containing rings can be individually saturated or unsaturated.
Such compounds are known for example from EP 2495266 A1, WO-2011/157742 or WO 2011/087841 as randomizer/polar agent/modifier in the preparation of high styrene high vinyl solution-based styrene-butadiene rubber or from EP 183538 A1 as a reactive diluent in epoxy resins.